The present invention relates in general to measuring technology and has particular reference to a differential eddy-current transducer.
The present invention can find application in mechanical engineering for non-destructive testing of current-conducting and/or ferromagnetic materials and products, namely, for measuring a distance to the surface, thickness of sheeting and coatings, dimensions of articles, as well as for control over production processes, in flaw detection, and in other fields of engineering and technology.
At present there are numerous constructions of differential eddy-current transducers extensively used in diverse apparatus and instruments, and in measuring systems.
Most of the heretofore-known eddy-current transducers are characterized by the presence of an error in the results of measurements, the so-called additional error, caused by external factors. Many of the improvements in the construction of the known eddy-current transducers are aimed at increasing the accuracy of measurements by minimizing the additional error; nevertheless, at present this problem is far from being solved completely.
Known in the art presently is a differential eddy-current transducer for measuring mechanical (non-electrical) quantities (cf. DE #3,817,371 A1) which comprises two coils and a ferromagnetic core movable inside said coils and connected to the object under control. Depending on the position assumed by the core the inductance values of the coils are varied, a common lead-out of said coils being connected to one of the inputs of an LC-oscillator while two other lead-outs thereof are alternately connected, via an analog multiplexer, to a second input of said LC-oscillator. The oscillation frequency of the self-excited oscillator depends on the inductance value of the LC-circuit, i.e., on the fact which of the coils at a given instant of time is connected to the input thereof. The frequency of the output signal of the LC-oscillator is determined after connecting each of the two coils, whereupon a microcomputer calculates, against the difference between frequencies, the value of the quantity measured by the transducer.
The eddy-current transducer discussed before is characterized by a low operating speed, necessity to make use of multi-digit counters (since its deviation is relatively low), restricted resolution, as well as reasonably high complexity and, accordingly, high cost of hardware implementation.
Known in prior art is an eddy-current measuring system (cf. U.S. Pat. No. 5,541,510 A) for non-destructive testing of electrically conducting and/or ferromagnetic materials and products, comprising a generator supplying power to an impedance network, an amplitude detector, a phase detector, demodulators, a computing unit, and an eddy-current transducer having one search coil and establishing a resonant circuit together with a parallel-connected capacitor. Said measuring system enables measuring two parameters pertaining to the object under test. However, it is more than two parameters that actually affect the eddy-current transducer in this case.
For instance, when measuring a distance to the object being tested and a linear dimension (thickness) thereof, a possible change in at least two out of the three other parameters occur to be non-compensated, viz, conductance, magnetic permeability, and temperature, as well as, probably the rest of the parameters of the object involved (its linear dimensions inclusive).
Effect of said factors on the results of measurements is partly eliminated in an embodiment of the system comprising two differentially connected eddy-current transducers. However, such a variant of the system involves stricter requirements imposed on the eddy-current transducers used, that is, as to similar dimensions and electric parameters, which is hard-to-attain due to technological spread in characteristics.
Furthermore, it is necessary that both of the eddy-current transducers be positioned very precisely so that they are in a similar position with respect to the object under test and/or under similar environmental conditions.
Non-identity of the eddy-current transducers and their arrangement results in an incomplete compensation for change in uncontrollable factors and, accordingly, leads to errors in the results of measurements.
In addition, processing of signals generated by eddy-current transducers necessitates the use of a costly instrument amplifier. The aforesaid peculiar features of the system add to its complexity and cost.
For electrically connecting eddy-current and inductive transducers there are most extensively used bridge circuits, wherein one of the bridge diagonals is power-supplied from a source of sinusoidal voltage, and the other bridge diagonal is connected to the inputs of a differential amplifier after which an alternating voltage is subjected to phase-sensitive detection and filtration.
Such circuit designs are characterized by sophisticated balancing, certain non-linearity of an unbalanced bridge, temporal and temperature instability, influence of electromagnetic interference protection against which involves the use of, e.g., expensive coaxial cable.
The closest to the herein-proposed transducer is a differential eddy-current transducer described in a prospectus of the Kaman Instrumentation (USA) entitled xe2x80x9cThe Measuring Solution Handbookxe2x80x9d, 1999, p.6 (also pp. 4 and 5), which transducer is also based on a bridge circuit.
Said known eddy-current transducer comprises a primary detector incorporating two similar search coils and an electronic unit comprising two capacitive and two resistive elements. Each of the search coils is shunted by a capacitive element together with which said coil forms a parallel resonant circuit. Both of such circuits are cut into adjacent arms of the bridge whose other two arms are in effect the resistive elements. The common point of the windings is grounded and an alternating voltage is applied to the common point of the resistive elements. An output signal generated by the circuit is picked off the common points of the resistors and coils.
To compensate for temperature instability is possible only in case of a balanced bridge, i.e., only with a fixed position of the object under test and an invariable value of the electromagnetic parameters of the object (that is, electrical conductance and magnetic permeability).
An incomplete identity of the parameters of the search coils and the presence of a spread in the parameters of the transducer circuitry components hampers selecting the capacitive elements for the resonant circuits and resistive elements for balancing the bridge arms. Hence complete balancing is practically impossible due to the fact that the bridge arms are formed by dissimilar elements. Even when the bridge is amplitude-balanced, a phase shift occurs which results, in case of phase-sensitive demodulation, in a xe2x80x9czero-driftxe2x80x9d error, affected resolution and temperature instability of the output signal.
An upset bridge balance occurs in the course of measurements when the object under test produces an unsymmetrical effect upon the search coils, which also tells negatively on the accuracy of the results of measurement.
And finally, further conversion of a pickup signal requires a differential (instrument) amplifier which complicates measuring equipment as a whole and adds to the cost thereof.
It is an essential object of the invention to enhance the accuracy of measurements by compensating for the additional error caused by external factors.
It is another object of the invention to enhance the resolving power and interference immunity of a differential eddy-current transducer.
The foregoing object is accomplished due to the fact that in a differential eddy-current transducer comprising a primary detector which is adapted to co-operate with the object under test and incorporates two similar search coils, each of which has a first output and a second output, a voltage being applied to said second output, and an electronic unit electrically connected to said primary detector, said electronic unit comprises a resistive element, a capacitive element and a signal amplifier at the output of which an output signal is shaped, according to the invention, said primary detector further comprises an additional coil having a first lead and a second lead through which leads said coil is connected in series aiding to the first outputs of the first and second search coils, respectively, the resistive element of said electronic unit appears as a potentiometer, and the signal amplifier appears as an operational amplifier having its non-inverting input grounded and its inverting input connected, via the capacitive element, to a midpoint lead of the potentiometer whose first and second leads are connected, respectively, to the first and second leads of the additional coil and to the first leads of the search coils, to the second leads of which an alternating voltage is applied in antiphase.
It is due to the herein-proposed technical solution that the present differential eddy-current transducer allows enhancing the accuracy of measurements due to compensating for temperature instability both under conditions of a symmetrical (similar) effect produced by the object under test on the search coils and in case of a non-symmetrical effect produced on the search coils and, moreover, when the object under test influences one coil only.
The additional coil is exposed to the same physical conditions as the search coils, whereby its impedance varies approximately to the same extent as from external factors. On account of a ratio-metering connection circuit of the coils, an output signal of the differential eddy-current transducer is directly proportional either to the ratio between the impedances of the search coil or the impedance coupled by the object under test, and the impedance of the additional coil, whereby an adverse effect of an ambient temperature on the accuracy of measurement is compensated for considerably.
It is important that the impedance values of the first and second search coils be equal to each other, and the impedance value of the additional coil be much in excess of that of each of the search coils, while the Q-factor of the first and second search coils and of the additional coil be equal to one another in the absence of the object under test.
It is reasonable, from the viewpoint of construction arrangement, that the differential eddy-current transducer comprises a first coil former on which the first search coil, the additional coil, and the second search coil are arranged in series and coaxially.
Depending on the characteristics of the controlled parameters of the object under test (i.e., air gap, thickness, diameter, length, conductance, magnetic permeability), at least one of said search coils is adapted for the object under test to be positioned nearby it.
In other embodiments of the invention the object under test may be disposed in an axial bore provided in the first coil former.
Whenever measurement is to be taken of a displacement performed by the object under test with respect to a certain fixed point, or of the length thereof, the axial length of the additional coil exceeds the length of the object under test with allowance for possible linear displacements thereof inside the axial bore of the first coil former.
Whenever the cross-sectional dimension of the object under test is to be measured or its electromagnetic properties are to be found, a total axial length of all of said coils should be less than the length of the object under test.
The present invention allows of measuring the parameters of diversely shaped objects.
In particular, for checking the wall thickness and/or the concentricity of the profile of a cylindrical shell, the additional coil is made up of two sections connected in series aiding to each other. The differential eddy-current transducer further comprises a second coil former whereon the first search coil and the first section of the additional coil are arranged, and a third coil whereon the second search coil and the second section of the additional coil are disposed, said first and said second search coils being adapted for the object under test to be interposed therebetween.
Such coils may have various predetermined configurations so as to suit the shape of the object under test.
Whenever the object under test is disposed outside the primary detector, a maximum cross-sectional dimension of the additional coil should be such that half the aforementioned dimension be less than the least possible axial distance from the additional coil to the object under test.
Such a cross-sectional dimension is selected on account of the fact that the object under test produces a substantial effect on the coil up to the aforesaid distance being approximately equal to half the coil cross-sectional dimension.
It is advantageous that the axial length of the additional coil should exceed the length of the object under test with allowance for its possible linear displacements within the axial bore of the first coil former, and that a total axial length of all of said coils be less than the length of the object under test.
It is expedient that the electronic unit should comprise a second capacitive element electrically connected to the potentiometer midpoint lead and to the output of the operational amplifier.
Provision of said second capacitive element adds to the reactive component of the impedance of the resonant circuit and thereby increases the Q-factor thereof.
It is favorable that the first and second search coils, the first and second capacitive elements, the potentiometer and the operational amplifier should have those parameters with which the first and second search coils and connected thereto the potentiometer, the first and second capacitive elements, and the operational amplifier form a series-connected resonant circuit having a Q-factor of from 10 to 20 with no object under test.
With no object under test the measuring circuit is tuned to resonance with the driving frequency and the impedance of the measuring circuit is changed under the effect of the object under test and hence the natural frequency of the circuit is changed, too, while high Q-factor value of the resonant circuit adds to the response of the present eddy-current transducer.
Further objects of the present invention will become apparent hereinafter from a detailed description of some specific embodiments thereof to be read with reference to the accompanying drawings.